[WebNN EP] Fix bug for PRelu on CPU backend. (#17543)

### Description
WebNN CPU backend expects slope of PRelu to be a static value. For now,
we will not support it.


### Motivation and Context
Fallback this case to pass the CI.

onnxruntime/core/providers/webnn/builders/impl/binary_op_builder.cc

@@ -18,6 +18,10 @@ class BinaryOpBuilder : public BaseOpBuilder {
  private:
   Status AddToModelBuilderImpl(ModelBuilder& model_builder, const Node& node,
                                const logging::Logger& logger) const override ORT_MUST_USE_RESULT;
+
+  // Operator support related.
+  bool IsOpSupportedImpl(const InitializedTensorSet& initializers, const Node& node,
+                         const WebnnDeviceType device_type, const logging::Logger& logger) const override;
 };
 
 // Add operator related.
@@ -50,6 +54,24 @@ Status BinaryOpBuilder::AddToModelBuilderImpl(ModelBuilder& model_builder, const
   return Status::OK();
 }
 
+bool BinaryOpBuilder::IsOpSupportedImpl(const InitializedTensorSet& initializers,
+                                        const Node& node,
+                                        const WebnnDeviceType device_type,
+                                        const logging::Logger& logger) const {
+  const auto& input_defs = node.InputDefs();
+  const auto& op_type = node.OpType();
+
+  // XNNPACK prelu operator expects slope to be a static value.
+  // https://github.com/google/XNNPACK/issues/4692
+  // TODO: Remove this check after it is solved.
+  if (op_type == "PRelu" && !Contains(initializers, input_defs[1]->Name()) && device_type == WebnnDeviceType::CPU) {
+    LOGS(logger, VERBOSE) << "The second input (slope) for PRelu must be a constant initializer for WebNN CPU backend.";
+    return false;
+  }
+
+  return true;
+}
+
 void CreateBinaryOpBuilder(const std::string& op_type, OpBuilderRegistrations& op_registrations) {
   if (op_registrations.op_builder_map.count(op_type) > 0)
     return;